Allow iDEAL tokens saving when SEPA is disabled

[wjrosa]

Fixes #3465

Changes proposed in this Pull Request:

iDEAL tokens are stored as SEPA tokens. Because of that, when SEPA is disabled, it is not possible to add or list saved iDEAL tokens.

This PR adds SEPA to the list of valid payment method tokens when listing saved methods to fix the issue.

Testing instructions

• Checkout and build this branch on your test environment (fix/allow-ideal-tokens-saving-when-sepa-is-disabled)
• Connect your Stripe account
• Set your store currency to EUR
• Enable the iDEAL payment method in settings (wp-admin/admin.php?page=wc-settings&tab=checkout&section=stripe&panel=methods). Keep SEPA disabled
• As a shopper, go to the Add Payment Method page (my-account/add-payment-method/)
• Try to add a new iDEAL method
• Confirm that the payment method is saved and is correctly listed:
  

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• Covered with tests (or have a good reason not to test in description ☝️)
• Added changelog entry in both changelog.txt and readme.txt (or does not apply)
• Tested on mobile (or does not apply)

Post merge

• Added testing instructions to the Release Testing Instructions wiki page (or does not apply)
• Added the needs docs label (or does not apply)
• Included this PR in the Release Thread scope (or does not apply)

[Mayisha]

Working as described 👍

I came across one edge case when I tested the following steps-

• Enable iDeal and SEPA.
• From My Account > Payment methods page, add a SEPA method. The method is added to the list.
• Now disable SEPA.
• Refresh the My Account > Payment methods. Notice that the added SEPA method shows up in the list.

Though I have documented the issue above, I think this is a very unlikely scenario for any site. So I am approving the PR as it fixes the original issue.

[wjrosa]

Working as described 👍

I came across one edge case when I tested the following steps-

• Enable iDeal and SEPA.
• From My Account > Payment methods page, add a SEPA method. The method is added to the list.
• Now disable SEPA.
• Refresh the My Account > Payment methods. Notice that the added SEPA method shows up in the list.

Though I have documented the issue above, I think this is a very unlikely scenario for any site. So I am approving the PR as it fixes the original issue.

Thanks for the review, Mayisha!

From what I could understand from the docs, both tokens are basically the same. So, SEPA tokens should be usable as iDEAL tokens and vice-versa.